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  <div class="section" id="extending-pytorch">
<h1>Extending PyTorch<a class="headerlink" href="#extending-pytorch" title="Permalink to this headline">¶</a></h1>
<p>In this note we’ll cover ways of extending <a class="reference internal" href="../nn.html#module-torch.nn" title="torch.nn"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch.nn</span></code></a>,
<a class="reference internal" href="../autograd.html#module-torch.autograd" title="torch.autograd"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch.autograd</span></code></a>, <a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a>, and writing custom C extensions utilizing our C
libraries.</p>
<div class="section" id="extending-torch-autograd">
<h2>Extending <a class="reference internal" href="../autograd.html#module-torch.autograd" title="torch.autograd"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch.autograd</span></code></a><a class="headerlink" href="#extending-torch-autograd" title="Permalink to this headline">¶</a></h2>
<p>Adding operations to <a class="reference internal" href="../autograd.html#module-torch.autograd" title="torch.autograd"><code class="xref py py-mod docutils literal notranslate"><span class="pre">autograd</span></code></a> requires implementing a new
<a class="reference internal" href="../autograd.html#torch.autograd.Function" title="torch.autograd.Function"><code class="xref py py-class docutils literal notranslate"><span class="pre">Function</span></code></a> subclass for each operation. Recall that <a class="reference internal" href="../autograd.html#torch.autograd.Function" title="torch.autograd.Function"><code class="xref py py-class docutils literal notranslate"><span class="pre">Function</span></code></a> s
are what <a class="reference internal" href="../autograd.html#module-torch.autograd" title="torch.autograd"><code class="xref py py-mod docutils literal notranslate"><span class="pre">autograd</span></code></a> uses to compute the results and gradients, and
encode the operation history. Every new function requires you to implement 2 methods:</p>
<ul class="simple">
<li><p><a class="reference internal" href="../autograd.html#torch.autograd.Function.forward" title="torch.autograd.Function.forward"><code class="xref py py-meth docutils literal notranslate"><span class="pre">forward()</span></code></a> - the code that performs the operation. It can take
as many arguments as you want, with some of them being optional, if you
specify the default values. All kinds of Python objects are accepted here.
<code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code> arguments that track history (i.e., with
<code class="docutils literal notranslate"><span class="pre">requires_grad=True</span></code>) will be converted to ones that don’t track history
before the call, and their use will be registered in the graph. Note that this
logic won’t traverse lists/dicts/any other data structures and will only
consider <code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code> s that are direct arguments to the call. You can
return either a single <code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code> output, or a <a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#tuple" title="(in Python v3.8)"><code class="xref py py-class docutils literal notranslate"><span class="pre">tuple</span></code></a> of
<code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code> s if there are multiple outputs. Also, please refer to the
docs of <a class="reference internal" href="../autograd.html#torch.autograd.Function" title="torch.autograd.Function"><code class="xref py py-class docutils literal notranslate"><span class="pre">Function</span></code></a> to find descriptions of useful methods that can be
called only from <a class="reference internal" href="../autograd.html#torch.autograd.Function.forward" title="torch.autograd.Function.forward"><code class="xref py py-meth docutils literal notranslate"><span class="pre">forward()</span></code></a>.</p></li>
<li><p><a class="reference internal" href="../autograd.html#torch.autograd.Function.backward" title="torch.autograd.Function.backward"><code class="xref py py-meth docutils literal notranslate"><span class="pre">backward()</span></code></a> - gradient formula. It will be given
as many <code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code> arguments as there were outputs, with each of them
representing gradient w.r.t. that output. It should return as many
<code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code> s as there were inputs, with each of them containing the
gradient w.r.t. its corresponding input. If your inputs didn’t require
gradient (<code class="xref py py-attr docutils literal notranslate"><span class="pre">needs_input_grad</span></code> is a tuple of booleans indicating
whether each input needs gradient computation), or were non-<code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code>
objects, you can return <code class="xref py py-class docutils literal notranslate"><span class="pre">None</span></code>. Also, if you have optional
arguments to <a class="reference internal" href="../autograd.html#torch.autograd.Function.forward" title="torch.autograd.Function.forward"><code class="xref py py-meth docutils literal notranslate"><span class="pre">forward()</span></code></a> you can return more gradients than there
were inputs, as long as they’re all <a class="reference external" href="https://docs.python.org/3/library/constants.html#None" title="(in Python v3.8)"><code class="docutils literal notranslate"><span class="pre">None</span></code></a>.</p></li>
</ul>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>It’s the user’s responsibility to use the special functions in the forward’s <cite>ctx</cite>
properly in order to ensure that the new <a class="reference internal" href="../autograd.html#torch.autograd.Function" title="torch.autograd.Function"><code class="xref py py-class docutils literal notranslate"><span class="pre">Function</span></code></a> works properly with
the autograd engine.</p>
<ul class="simple">
<li><p><a class="reference internal" href="../autograd.html#torch.autograd.function._ContextMethodMixin.save_for_backward" title="torch.autograd.function._ContextMethodMixin.save_for_backward"><code class="xref py py-meth docutils literal notranslate"><span class="pre">save_for_backward()</span></code></a> must be
used when saving input or output of the forward to be used later in the backward.</p></li>
<li><p><a class="reference internal" href="../autograd.html#torch.autograd.function._ContextMethodMixin.mark_dirty" title="torch.autograd.function._ContextMethodMixin.mark_dirty"><code class="xref py py-meth docutils literal notranslate"><span class="pre">mark_dirty()</span></code></a> must be used to
mark any input that is modified inplace by the forward function.</p></li>
<li><p><a class="reference internal" href="../autograd.html#torch.autograd.function._ContextMethodMixin.mark_non_differentiable" title="torch.autograd.function._ContextMethodMixin.mark_non_differentiable"><code class="xref py py-meth docutils literal notranslate"><span class="pre">mark_non_differentiable()</span></code></a> must
be used to tell the engine if an output is not differentiable.</p></li>
</ul>
</div>
<p>Below you can find code for a <code class="docutils literal notranslate"><span class="pre">Linear</span></code> function from <a class="reference internal" href="../nn.html#module-torch.nn" title="torch.nn"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch.nn</span></code></a>, with
additional comments:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># Inherit from Function</span>
<span class="k">class</span> <span class="nc">LinearFunction</span><span class="p">(</span><span class="n">Function</span><span class="p">):</span>

    <span class="c1"># Note that both forward and backward are @staticmethods</span>
    <span class="nd">@staticmethod</span>
    <span class="c1"># bias is an optional argument</span>
    <span class="k">def</span> <span class="nf">forward</span><span class="p">(</span><span class="n">ctx</span><span class="p">,</span> <span class="nb">input</span><span class="p">,</span> <span class="n">weight</span><span class="p">,</span> <span class="n">bias</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
        <span class="n">ctx</span><span class="o">.</span><span class="n">save_for_backward</span><span class="p">(</span><span class="nb">input</span><span class="p">,</span> <span class="n">weight</span><span class="p">,</span> <span class="n">bias</span><span class="p">)</span>
        <span class="n">output</span> <span class="o">=</span> <span class="nb">input</span><span class="o">.</span><span class="n">mm</span><span class="p">(</span><span class="n">weight</span><span class="o">.</span><span class="n">t</span><span class="p">())</span>
        <span class="k">if</span> <span class="n">bias</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
            <span class="n">output</span> <span class="o">+=</span> <span class="n">bias</span><span class="o">.</span><span class="n">unsqueeze</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span><span class="o">.</span><span class="n">expand_as</span><span class="p">(</span><span class="n">output</span><span class="p">)</span>
        <span class="k">return</span> <span class="n">output</span>

    <span class="c1"># This function has only a single output, so it gets only one gradient</span>
    <span class="nd">@staticmethod</span>
    <span class="k">def</span> <span class="nf">backward</span><span class="p">(</span><span class="n">ctx</span><span class="p">,</span> <span class="n">grad_output</span><span class="p">):</span>
        <span class="c1"># This is a pattern that is very convenient - at the top of backward</span>
        <span class="c1"># unpack saved_tensors and initialize all gradients w.r.t. inputs to</span>
        <span class="c1"># None. Thanks to the fact that additional trailing Nones are</span>
        <span class="c1"># ignored, the return statement is simple even when the function has</span>
        <span class="c1"># optional inputs.</span>
        <span class="nb">input</span><span class="p">,</span> <span class="n">weight</span><span class="p">,</span> <span class="n">bias</span> <span class="o">=</span> <span class="n">ctx</span><span class="o">.</span><span class="n">saved_tensors</span>
        <span class="n">grad_input</span> <span class="o">=</span> <span class="n">grad_weight</span> <span class="o">=</span> <span class="n">grad_bias</span> <span class="o">=</span> <span class="kc">None</span>

        <span class="c1"># These needs_input_grad checks are optional and there only to</span>
        <span class="c1"># improve efficiency. If you want to make your code simpler, you can</span>
        <span class="c1"># skip them. Returning gradients for inputs that don&#39;t require it is</span>
        <span class="c1"># not an error.</span>
        <span class="k">if</span> <span class="n">ctx</span><span class="o">.</span><span class="n">needs_input_grad</span><span class="p">[</span><span class="mi">0</span><span class="p">]:</span>
            <span class="n">grad_input</span> <span class="o">=</span> <span class="n">grad_output</span><span class="o">.</span><span class="n">mm</span><span class="p">(</span><span class="n">weight</span><span class="p">)</span>
        <span class="k">if</span> <span class="n">ctx</span><span class="o">.</span><span class="n">needs_input_grad</span><span class="p">[</span><span class="mi">1</span><span class="p">]:</span>
            <span class="n">grad_weight</span> <span class="o">=</span> <span class="n">grad_output</span><span class="o">.</span><span class="n">t</span><span class="p">()</span><span class="o">.</span><span class="n">mm</span><span class="p">(</span><span class="nb">input</span><span class="p">)</span>
        <span class="k">if</span> <span class="n">bias</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span> <span class="ow">and</span> <span class="n">ctx</span><span class="o">.</span><span class="n">needs_input_grad</span><span class="p">[</span><span class="mi">2</span><span class="p">]:</span>
            <span class="n">grad_bias</span> <span class="o">=</span> <span class="n">grad_output</span><span class="o">.</span><span class="n">sum</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>

        <span class="k">return</span> <span class="n">grad_input</span><span class="p">,</span> <span class="n">grad_weight</span><span class="p">,</span> <span class="n">grad_bias</span>
</pre></div>
</div>
<p>Now, to make it easier to use these custom ops, we recommend aliasing their
<code class="docutils literal notranslate"><span class="pre">apply</span></code> method:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">linear</span> <span class="o">=</span> <span class="n">LinearFunction</span><span class="o">.</span><span class="n">apply</span>
</pre></div>
</div>
<p>Here, we give an additional example of a function that is parametrized by
non-Tensor arguments:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">MulConstant</span><span class="p">(</span><span class="n">Function</span><span class="p">):</span>
    <span class="nd">@staticmethod</span>
    <span class="k">def</span> <span class="nf">forward</span><span class="p">(</span><span class="n">ctx</span><span class="p">,</span> <span class="n">tensor</span><span class="p">,</span> <span class="n">constant</span><span class="p">):</span>
        <span class="c1"># ctx is a context object that can be used to stash information</span>
        <span class="c1"># for backward computation</span>
        <span class="n">ctx</span><span class="o">.</span><span class="n">constant</span> <span class="o">=</span> <span class="n">constant</span>
        <span class="k">return</span> <span class="n">tensor</span> <span class="o">*</span> <span class="n">constant</span>

    <span class="nd">@staticmethod</span>
    <span class="k">def</span> <span class="nf">backward</span><span class="p">(</span><span class="n">ctx</span><span class="p">,</span> <span class="n">grad_output</span><span class="p">):</span>
        <span class="c1"># We return as many input gradients as there were arguments.</span>
        <span class="c1"># Gradients of non-Tensor arguments to forward must be None.</span>
        <span class="k">return</span> <span class="n">grad_output</span> <span class="o">*</span> <span class="n">ctx</span><span class="o">.</span><span class="n">constant</span><span class="p">,</span> <span class="kc">None</span>
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Inputs to <code class="docutils literal notranslate"><span class="pre">backward</span></code>, i.e., <code class="xref py py-attr docutils literal notranslate"><span class="pre">grad_output</span></code>, can also be Tensors that
track history. So if <code class="docutils literal notranslate"><span class="pre">backward</span></code> is implemented with differentiable
operations, (e.g., invocation of another custom
<code class="xref py py-class docutils literal notranslate"><span class="pre">function</span></code>), higher order derivatives will work.</p>
</div>
<p>You probably want to check if the backward method you implemented actually
computes the derivatives of your function. It is possible by comparing with
numerical approximations using small finite differences:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">torch.autograd</span> <span class="kn">import</span> <span class="n">gradcheck</span>

<span class="c1"># gradcheck takes a tuple of tensors as input, check if your gradient</span>
<span class="c1"># evaluated with these tensors are close enough to numerical</span>
<span class="c1"># approximations and returns True if they all verify this condition.</span>
<span class="nb">input</span> <span class="o">=</span> <span class="p">(</span><span class="n">torch</span><span class="o">.</span><span class="n">randn</span><span class="p">(</span><span class="mi">20</span><span class="p">,</span><span class="mi">20</span><span class="p">,</span><span class="n">dtype</span><span class="o">=</span><span class="n">torch</span><span class="o">.</span><span class="n">double</span><span class="p">,</span><span class="n">requires_grad</span><span class="o">=</span><span class="kc">True</span><span class="p">),</span> <span class="n">torch</span><span class="o">.</span><span class="n">randn</span><span class="p">(</span><span class="mi">30</span><span class="p">,</span><span class="mi">20</span><span class="p">,</span><span class="n">dtype</span><span class="o">=</span><span class="n">torch</span><span class="o">.</span><span class="n">double</span><span class="p">,</span><span class="n">requires_grad</span><span class="o">=</span><span class="kc">True</span><span class="p">))</span>
<span class="n">test</span> <span class="o">=</span> <span class="n">gradcheck</span><span class="p">(</span><span class="n">linear</span><span class="p">,</span> <span class="nb">input</span><span class="p">,</span> <span class="n">eps</span><span class="o">=</span><span class="mf">1e-6</span><span class="p">,</span> <span class="n">atol</span><span class="o">=</span><span class="mf">1e-4</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">test</span><span class="p">)</span>
</pre></div>
</div>
<p>See <a class="reference internal" href="../autograd.html#grad-check"><span class="std std-ref">Numerical gradient checking</span></a> for more details on finite-difference gradient comparisons.</p>
</div>
<div class="section" id="extending-torch-nn">
<h2>Extending <a class="reference internal" href="../nn.html#module-torch.nn" title="torch.nn"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch.nn</span></code></a><a class="headerlink" href="#extending-torch-nn" title="Permalink to this headline">¶</a></h2>
<p><a class="reference internal" href="../nn.html#module-torch.nn" title="torch.nn"><code class="xref py py-mod docutils literal notranslate"><span class="pre">nn</span></code></a> exports two kinds of interfaces - modules and their functional
versions. You can extend it in both ways, but we recommend using modules for
all kinds of layers, that hold any parameters or buffers, and recommend using
a functional form parameter-less operations like activation functions, pooling,
etc.</p>
<p>Adding a functional version of an operation is already fully covered in the
section above.</p>
<div class="section" id="adding-a-module">
<h3>Adding a <a class="reference internal" href="../nn.html#torch.nn.Module" title="torch.nn.Module"><code class="xref py py-class docutils literal notranslate"><span class="pre">Module</span></code></a><a class="headerlink" href="#adding-a-module" title="Permalink to this headline">¶</a></h3>
<p>Since <a class="reference internal" href="../nn.html#module-torch.nn" title="torch.nn"><code class="xref py py-mod docutils literal notranslate"><span class="pre">nn</span></code></a> heavily utilizes <a class="reference internal" href="../autograd.html#module-torch.autograd" title="torch.autograd"><code class="xref py py-mod docutils literal notranslate"><span class="pre">autograd</span></code></a>, adding a new
<a class="reference internal" href="../nn.html#torch.nn.Module" title="torch.nn.Module"><code class="xref py py-class docutils literal notranslate"><span class="pre">Module</span></code></a> requires implementing a <a class="reference internal" href="../autograd.html#torch.autograd.Function" title="torch.autograd.Function"><code class="xref py py-class docutils literal notranslate"><span class="pre">Function</span></code></a>
that performs the operation and can compute the gradient. From now on let’s
assume that we want to implement a <code class="docutils literal notranslate"><span class="pre">Linear</span></code> module and we have the function
implemented as in the listing above. There’s very little code required to
add this. Now, there are two functions that need to be implemented:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">__init__</span></code> (<em>optional</em>) - takes in arguments such as kernel sizes, numbers
of features, etc. and initializes parameters and buffers.</p></li>
<li><p><a class="reference internal" href="../nn.html#torch.nn.Module.forward" title="torch.nn.Module.forward"><code class="xref py py-meth docutils literal notranslate"><span class="pre">forward()</span></code></a> - instantiates a <a class="reference internal" href="../autograd.html#torch.autograd.Function" title="torch.autograd.Function"><code class="xref py py-class docutils literal notranslate"><span class="pre">Function</span></code></a> and
uses it to perform the operation. It’s very similar to a functional wrapper
shown above.</p></li>
</ul>
<p>This is how a <code class="docutils literal notranslate"><span class="pre">Linear</span></code> module can be implemented:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Linear</span><span class="p">(</span><span class="n">nn</span><span class="o">.</span><span class="n">Module</span><span class="p">):</span>
    <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">input_features</span><span class="p">,</span> <span class="n">output_features</span><span class="p">,</span> <span class="n">bias</span><span class="o">=</span><span class="kc">True</span><span class="p">):</span>
        <span class="nb">super</span><span class="p">(</span><span class="n">Linear</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="fm">__init__</span><span class="p">()</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">input_features</span> <span class="o">=</span> <span class="n">input_features</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">output_features</span> <span class="o">=</span> <span class="n">output_features</span>

        <span class="c1"># nn.Parameter is a special kind of Tensor, that will get</span>
        <span class="c1"># automatically registered as Module&#39;s parameter once it&#39;s assigned</span>
        <span class="c1"># as an attribute. Parameters and buffers need to be registered, or</span>
        <span class="c1"># they won&#39;t appear in .parameters() (doesn&#39;t apply to buffers), and</span>
        <span class="c1"># won&#39;t be converted when e.g. .cuda() is called. You can use</span>
        <span class="c1"># .register_buffer() to register buffers.</span>
        <span class="c1"># nn.Parameters require gradients by default.</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">weight</span> <span class="o">=</span> <span class="n">nn</span><span class="o">.</span><span class="n">Parameter</span><span class="p">(</span><span class="n">torch</span><span class="o">.</span><span class="n">Tensor</span><span class="p">(</span><span class="n">output_features</span><span class="p">,</span> <span class="n">input_features</span><span class="p">))</span>
        <span class="k">if</span> <span class="n">bias</span><span class="p">:</span>
            <span class="bp">self</span><span class="o">.</span><span class="n">bias</span> <span class="o">=</span> <span class="n">nn</span><span class="o">.</span><span class="n">Parameter</span><span class="p">(</span><span class="n">torch</span><span class="o">.</span><span class="n">Tensor</span><span class="p">(</span><span class="n">output_features</span><span class="p">))</span>
        <span class="k">else</span><span class="p">:</span>
            <span class="c1"># You should always register all possible parameters, but the</span>
            <span class="c1"># optional ones can be None if you want.</span>
            <span class="bp">self</span><span class="o">.</span><span class="n">register_parameter</span><span class="p">(</span><span class="s1">&#39;bias&#39;</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>

        <span class="c1"># Not a very smart way to initialize weights</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">weight</span><span class="o">.</span><span class="n">data</span><span class="o">.</span><span class="n">uniform_</span><span class="p">(</span><span class="o">-</span><span class="mf">0.1</span><span class="p">,</span> <span class="mf">0.1</span><span class="p">)</span>
        <span class="k">if</span> <span class="n">bias</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
            <span class="bp">self</span><span class="o">.</span><span class="n">bias</span><span class="o">.</span><span class="n">data</span><span class="o">.</span><span class="n">uniform_</span><span class="p">(</span><span class="o">-</span><span class="mf">0.1</span><span class="p">,</span> <span class="mf">0.1</span><span class="p">)</span>

    <span class="k">def</span> <span class="nf">forward</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="nb">input</span><span class="p">):</span>
        <span class="c1"># See the autograd section for explanation of what happens here.</span>
        <span class="k">return</span> <span class="n">LinearFunction</span><span class="o">.</span><span class="n">apply</span><span class="p">(</span><span class="nb">input</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">weight</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">bias</span><span class="p">)</span>

    <span class="k">def</span> <span class="nf">extra_repr</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="c1"># (Optional)Set the extra information about this module. You can test</span>
        <span class="c1"># it by printing an object of this class.</span>
        <span class="k">return</span> <span class="s1">&#39;input_features=</span><span class="si">{}</span><span class="s1">, output_features=</span><span class="si">{}</span><span class="s1">, bias=</span><span class="si">{}</span><span class="s1">&#39;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span>
            <span class="bp">self</span><span class="o">.</span><span class="n">input_features</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">output_features</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">bias</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span>
        <span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="extending-torch">
<h2>Extending <a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a><a class="headerlink" href="#extending-torch" title="Permalink to this headline">¶</a></h2>
<p>You can create custom types that emulate <code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code> by defining a custom
class with methods that match <code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code>. But what if you want to be able
to pass these types to functions like <a class="reference internal" href="../torch.html#torch.add" title="torch.add"><code class="xref py py-func docutils literal notranslate"><span class="pre">torch.add()</span></code></a> in the top-level
<a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a> namespace that accept <code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code> operands?</p>
<p>If your custom python type defines a method named <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code>, PyTorch
will invoke your <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> implementation when an instance of your
custom class is passed to a function in the <a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a> namespace. This makes
it possible to define custom implementations for any of the functions in the
<a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a> namespace which your <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> implementation can call,
allowing your users to make use of your custom type with existing PyTorch
workflows that they have already written for <code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code>. This works with
“duck” types that are unrelated to <code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code> as well as user-defined
subclasses of <code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code>.</p>
<div class="section" id="extending-torch-with-a-tensor-like-type">
<h3>Extending <a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a> with a <code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code>-like type<a class="headerlink" href="#extending-torch-with-a-tensor-like-type" title="Permalink to this headline">¶</a></h3>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>This functionality is inspired by the NumPy <code class="docutils literal notranslate"><span class="pre">__array_function__</span></code>
protocol. See <a class="reference external" href="https://docs.scipy.org/doc/numpy/user/basics.dispatch.html#basics-dispatch">the NumPy documentation</a>
and <a class="reference external" href="https://numpy.org/neps/nep-0018-array-function-protocol.html">NEP-0018</a> for
more details.</p>
</div>
<p>To make this concrete, let’s begin with a simple example that illustrates the
API dispatch mechanism. We’ll create a custom type that represents a 2D scalar
tensor, parametrized by the order <code class="docutils literal notranslate"><span class="pre">N</span></code> and value along the diagonal entries,
<code class="docutils literal notranslate"><span class="pre">value</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">ScalarTensor</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
   <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">N</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
       <span class="bp">self</span><span class="o">.</span><span class="n">_N</span> <span class="o">=</span> <span class="n">N</span>
       <span class="bp">self</span><span class="o">.</span><span class="n">_value</span> <span class="o">=</span> <span class="n">value</span>

   <span class="k">def</span> <span class="fm">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
       <span class="k">return</span> <span class="s2">&quot;DiagonalTensor(N=</span><span class="si">{}</span><span class="s2">, value=</span><span class="si">{}</span><span class="s2">)&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">_N</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">_value</span><span class="p">)</span>

   <span class="k">def</span> <span class="nf">tensor</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
       <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_value</span> <span class="o">*</span> <span class="n">torch</span><span class="o">.</span><span class="n">eye</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">_N</span><span class="p">)</span>
</pre></div>
</div>
<p>This first iteration of the design isn’t very useful. The main functionality of
<code class="docutils literal notranslate"><span class="pre">ScalarTensor</span></code> is to provide a more compact string representation of a scalar
tensor than in the base tensor class:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">d</span> <span class="o">=</span> <span class="n">ScalarTensor</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">d</span>
<span class="go">ScalarTensor(N=5, value=2)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">d</span><span class="o">.</span><span class="n">tensor</span><span class="p">()</span>
<span class="go">tensor([[2., 0., 0., 0., 0.],</span>
<span class="go">        [0., 2., 0., 0., 0.],</span>
<span class="go">        [0., 0., 2., 0., 0.],</span>
<span class="go">        [0., 0., 0., 2., 0.],</span>
<span class="go">        [0., 0., 0., 0., 2.]])</span>
</pre></div>
</div>
<p>If we try to use this object with the <a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a> API, we will run
into issues:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">import</span> <span class="nn">torch</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">torch</span><span class="o">.</span><span class="n">mean</span><span class="p">(</span><span class="n">d</span><span class="p">)</span>
<span class="go">TypeError: mean(): argument &#39;input&#39; (position 1) must be Tensor, not ScalarTensor</span>
</pre></div>
</div>
<p>Adding a <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> implementation to <code class="docutils literal notranslate"><span class="pre">ScalarTensor</span></code> makes it
possible for the above operation to succeed. Let’s re-do our implementation,
this time adding a <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> implementation:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">HANDLED_FUNCTIONS</span> <span class="o">=</span> <span class="p">{}</span>
<span class="k">class</span> <span class="nc">ScalarTensor</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
    <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">N</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">_N</span> <span class="o">=</span> <span class="n">N</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">_value</span> <span class="o">=</span> <span class="n">value</span>

    <span class="k">def</span> <span class="fm">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="s2">&quot;DiagonalTensor(N=</span><span class="si">{}</span><span class="s2">, value=</span><span class="si">{}</span><span class="s2">)&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">_N</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">_value</span><span class="p">)</span>

    <span class="k">def</span> <span class="nf">tensor</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_value</span> <span class="o">*</span> <span class="n">torch</span><span class="o">.</span><span class="n">eye</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">_N</span><span class="p">)</span>

    <span class="k">def</span> <span class="nf">__torch_function__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">func</span><span class="p">,</span> <span class="n">types</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(),</span> <span class="n">kwargs</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
        <span class="k">if</span> <span class="n">kwargs</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
            <span class="n">kwargs</span> <span class="o">=</span> <span class="p">{}</span>
        <span class="k">if</span> <span class="n">func</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">HANDLED_FUNCTIONS</span> <span class="ow">or</span> <span class="ow">not</span> <span class="nb">all</span><span class="p">(</span>
            <span class="nb">issubclass</span><span class="p">(</span><span class="n">t</span><span class="p">,</span> <span class="p">(</span><span class="n">torch</span><span class="o">.</span><span class="n">Tensor</span><span class="p">,</span> <span class="n">ScalarTensor</span><span class="p">))</span>
            <span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">types</span>
        <span class="p">):</span>
            <span class="k">return</span> <span class="bp">NotImplemented</span>
        <span class="k">return</span> <span class="n">HANDLED_FUNCTIONS</span><span class="p">[</span><span class="n">func</span><span class="p">](</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> method takes four arguments: <code class="docutils literal notranslate"><span class="pre">func</span></code>, a reference
to the torch API function that is being overridden, <code class="docutils literal notranslate"><span class="pre">types</span></code>, the list of
types of Tensor-likes that implement <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code>, <code class="docutils literal notranslate"><span class="pre">args</span></code>, the
tuple of arguments passed to the function, and <code class="docutils literal notranslate"><span class="pre">kwargs</span></code>, the dict of keyword
arguments passed to the function. It uses a global dispatch stable named
<code class="docutils literal notranslate"><span class="pre">HANDLED_FUNCTIONS</span></code> to store custom implementations. The keys of this
dictionary are functions in the <code class="docutils literal notranslate"><span class="pre">torch</span></code> namespace and the values are
implementations for <code class="docutils literal notranslate"><span class="pre">ScalarTensor</span></code>.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Using a global dispatch table is not a mandated part of the
<code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> API, it is just a useful design pattern for
structuring your override implementations.</p>
</div>
<p>This class definition isn’t quite enough to make <code class="docutils literal notranslate"><span class="pre">torch.mean</span></code> do the right
thing when we pass it a <code class="docutils literal notranslate"><span class="pre">ScalarTensor</span></code> – we also need to define an
implementation for <code class="docutils literal notranslate"><span class="pre">torch.mean</span></code> for <code class="docutils literal notranslate"><span class="pre">ScalarTensor</span></code> operands and add the
implementation to the <code class="docutils literal notranslate"><span class="pre">HANDLED_FUNCTIONS</span></code> dispatch table dictionary. One way
of doing this is to define a decorator:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">functools</span>
<span class="k">def</span> <span class="nf">implements</span><span class="p">(</span><span class="n">torch_function</span><span class="p">):</span>
    <span class="sd">&quot;&quot;&quot;Register a torch function override for ScalarTensor&quot;&quot;&quot;</span>
    <span class="nd">@functools</span><span class="o">.</span><span class="n">wraps</span><span class="p">(</span><span class="n">torch_function</span><span class="p">)</span>
    <span class="k">def</span> <span class="nf">decorator</span><span class="p">(</span><span class="n">func</span><span class="p">):</span>
        <span class="n">HANDLED_FUNCTIONS</span><span class="p">[</span><span class="n">torch_function</span><span class="p">]</span> <span class="o">=</span> <span class="n">func</span>
        <span class="k">return</span> <span class="n">func</span>
    <span class="k">return</span> <span class="n">decorator</span>
</pre></div>
</div>
<p>which can be applied to the implementation of our override:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nd">@implements</span><span class="p">(</span><span class="n">torch</span><span class="o">.</span><span class="n">mean</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">mean</span><span class="p">(</span><span class="nb">input</span><span class="p">):</span>
    <span class="k">return</span> <span class="nb">float</span><span class="p">(</span><span class="nb">input</span><span class="o">.</span><span class="n">_value</span><span class="p">)</span> <span class="o">/</span> <span class="nb">input</span><span class="o">.</span><span class="n">_N</span>
</pre></div>
</div>
<p>With this change we can now use <code class="docutils literal notranslate"><span class="pre">torch.mean</span></code> with <code class="docutils literal notranslate"><span class="pre">ScalarTensor</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">d</span> <span class="o">=</span> <span class="n">ScalarTensor</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">torch</span><span class="o">.</span><span class="n">mean</span><span class="p">(</span><span class="n">d</span><span class="p">)</span>
<span class="go">0.4</span>
</pre></div>
</div>
<p>Of course <code class="docutils literal notranslate"><span class="pre">torch.mean</span></code> is an example of the simplest kind of function to
override since it only takes one operand. We can use the same machinery to
override a function that takes more than one operand, any one of which might be
a tensor or tensor-like that defines <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code>, for example for
<a class="reference internal" href="../torch.html#torch.add" title="torch.add"><code class="xref py py-func docutils literal notranslate"><span class="pre">torch.add()</span></code></a>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">ensure_tensor</span><span class="p">(</span><span class="n">data</span><span class="p">):</span>
    <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">ScalarTensor</span><span class="p">):</span>
        <span class="k">return</span> <span class="n">data</span><span class="o">.</span><span class="n">tensor</span><span class="p">()</span>
    <span class="k">return</span> <span class="n">torch</span><span class="o">.</span><span class="n">as_tensor</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>

<span class="nd">@implements</span><span class="p">(</span><span class="n">torch</span><span class="o">.</span><span class="n">add</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">add</span><span class="p">(</span><span class="nb">input</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span>
   <span class="k">try</span><span class="p">:</span>
       <span class="k">if</span> <span class="nb">input</span><span class="o">.</span><span class="n">_N</span> <span class="o">==</span> <span class="n">other</span><span class="o">.</span><span class="n">_N</span><span class="p">:</span>
           <span class="k">return</span> <span class="n">ScalarTensor</span><span class="p">(</span><span class="nb">input</span><span class="o">.</span><span class="n">_N</span><span class="p">,</span> <span class="nb">input</span><span class="o">.</span><span class="n">_value</span> <span class="o">+</span> <span class="n">other</span><span class="o">.</span><span class="n">_value</span><span class="p">)</span>
       <span class="k">else</span><span class="p">:</span>
           <span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s2">&quot;Shape mismatch!&quot;</span><span class="p">)</span>
   <span class="k">except</span> <span class="ne">AttributeError</span><span class="p">:</span>
       <span class="k">return</span> <span class="n">torch</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">ensure_tensor</span><span class="p">(</span><span class="nb">input</span><span class="p">),</span> <span class="n">ensure_tensor</span><span class="p">(</span><span class="n">other</span><span class="p">))</span>
</pre></div>
</div>
<p>This version has a fast path for when both operands are <code class="docutils literal notranslate"><span class="pre">ScalarTensor</span></code>
instances and also a slower path which degrades to converting the data to
tensors when either operand is not a <code class="docutils literal notranslate"><span class="pre">ScalarTensor</span></code>. That makes the override
function correctly when either operand is a <code class="docutils literal notranslate"><span class="pre">ScalarTensor</span></code> or a regular
<code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">s</span> <span class="o">=</span> <span class="n">ScalarTensor</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">torch</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="n">s</span><span class="p">)</span>
<span class="go">DiagonalTensor(N=2, value=4)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">t</span> <span class="o">=</span> <span class="n">torch</span><span class="o">.</span><span class="n">tensor</span><span class="p">([[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">,],</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">]])</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">torch</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="n">t</span><span class="p">)</span>
<span class="go">tensor([[3., 1.],</span>
<span class="go">        [1., 3.]])</span>
</pre></div>
</div>
<p>Note that our implementation of <code class="docutils literal notranslate"><span class="pre">add</span></code> does not take <code class="docutils literal notranslate"><span class="pre">alpha</span></code> or <code class="docutils literal notranslate"><span class="pre">out</span></code> as
keyword arguments like <a class="reference internal" href="../torch.html#torch.add" title="torch.add"><code class="xref py py-func docutils literal notranslate"><span class="pre">torch.add()</span></code></a> does:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">torch</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="n">s</span><span class="p">,</span> <span class="n">alpha</span><span class="o">=</span><span class="mi">2</span><span class="p">)</span>
<span class="go">TypeError: add() got an unexpected keyword argument &#39;alpha&#39;</span>
</pre></div>
</div>
<p>For speed and flexibility the <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> dispatch mechanism does not
check that the signature of an override function matches the signature of the
function being overrided in the <a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a> API. For some applications ignoring
optional arguments would be fine but to ensure full compatibility with
<code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code>, user implementations of torch API functions should take care to
exactly emulate the API of the function that is being overrided.</p>
<p>Functions in the <a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a> API that do not have explicit overrides will
return <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code> from <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code>. If all operands with
<code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> defined on them return <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>, PyTorch will
raise a <code class="docutils literal notranslate"><span class="pre">TypeError</span></code>. This means that most of the time operations that do not
have explicit overrides for a type will raise a <code class="docutils literal notranslate"><span class="pre">TypeError</span></code> when an instance
of such a type is passed:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">torch</span><span class="o">.</span><span class="n">mul</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>
<span class="go">TypeError: no implementation found for &#39;torch.mul&#39; on types that</span>
<span class="go">implement __torch_function__: [ScalarTensor]</span>
</pre></div>
</div>
<p>In practice this means that if you would like to implement your overrides using
a <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> implementation along these lines, you will need to
explicitly implement the full <a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a> API or the entire subset of the API
that you care about for your use case. This may be a tall order as the full
<a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a> API is quite extensive.</p>
<p>Another option is to not return <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code> for operations that are not
handled but to instead pass a <code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code> to the original <a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a>
function when no override is available. For example, if we change our
implementation of <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> for <code class="docutils literal notranslate"><span class="pre">ScalarTensor</span></code> to the one below:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">__torch_function__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">func</span><span class="p">,</span> <span class="n">types</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(),</span> <span class="n">kwargs</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
    <span class="k">if</span> <span class="n">kwargs</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
        <span class="n">kwargs</span> <span class="o">=</span> <span class="p">{}</span>
    <span class="k">if</span> <span class="n">func</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">HANDLED_FUNCTIONS</span> <span class="ow">or</span> <span class="ow">not</span> <span class="nb">all</span><span class="p">(</span>
            <span class="nb">issubclass</span><span class="p">(</span><span class="n">t</span><span class="p">,</span> <span class="p">(</span><span class="n">torch</span><span class="o">.</span><span class="n">Tensor</span><span class="p">,</span> <span class="n">ScalarTensor</span><span class="p">))</span>
            <span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">types</span>
        <span class="p">):</span>
        <span class="n">args</span> <span class="o">=</span> <span class="p">[</span><span class="n">a</span><span class="o">.</span><span class="n">tensor</span><span class="p">()</span> <span class="k">if</span> <span class="nb">hasattr</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="s1">&#39;tensor&#39;</span><span class="p">)</span> <span class="k">else</span> <span class="n">a</span> <span class="k">for</span> <span class="n">a</span> <span class="ow">in</span> <span class="n">args</span><span class="p">]</span>
        <span class="k">return</span> <span class="n">func</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
    <span class="k">return</span> <span class="n">HANDLED_FUNCTIONS</span><span class="p">[</span><span class="n">func</span><span class="p">](</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
</pre></div>
</div>
<p>Then <a class="reference internal" href="../torch.html#torch.mul" title="torch.mul"><code class="xref py py-func docutils literal notranslate"><span class="pre">torch.mul()</span></code></a> will work correctly, although the return type will always
be a <code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code> rather than a <code class="xref py py-class docutils literal notranslate"><span class="pre">ScalarTensor</span></code>, even if both operands
are <code class="xref py py-class docutils literal notranslate"><span class="pre">ScalarTensor</span></code> instances:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">s</span> <span class="o">=</span> <span class="n">ScalarTensor</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">torch</span><span class="o">.</span><span class="n">mul</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="n">s</span><span class="p">)</span>
<span class="go">tensor([[4., 0.],</span>
<span class="go">        [0., 4.]])</span>
</pre></div>
</div>
<p>Also see the <code class="docutils literal notranslate"><span class="pre">MetadataTensor</span></code> example below for another variation on this
pattern but instead always returns a <code class="docutils literal notranslate"><span class="pre">MetadataTensor</span></code> to propagate metadata
through operations in the <a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a> API.</p>
</div>
<div class="section" id="extending-torch-with-a-tensor-wrapper-type">
<h3>Extending <a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a> with a <code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code> wrapper type<a class="headerlink" href="#extending-torch-with-a-tensor-wrapper-type" title="Permalink to this headline">¶</a></h3>
<p>Another useful case is a type that wraps a <code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code>, either as an
attribute or via subclassing. Below we implement a special case of this sort of
type, a <code class="docutils literal notranslate"><span class="pre">MetadataTensor</span></code> that attaches a dictionary of metadata to a
<code class="xref py py-class docutils literal notranslate"><span class="pre">Tensor</span></code> that is propagated through <a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a> operations. Since this
is a generic sort of wrapping for the full <a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a> API, we do not need to
individually implement each override so we can make the <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code>
implementation more permissive about what operations are allowed:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">MetadataTensor</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
    <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">data</span><span class="p">,</span> <span class="n">metadata</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">_t</span> <span class="o">=</span> <span class="n">torch</span><span class="o">.</span><span class="n">as_tensor</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">_metadata</span> <span class="o">=</span> <span class="n">metadata</span>

    <span class="k">def</span> <span class="fm">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="s2">&quot;Metadata:</span><span class="se">\n</span><span class="si">{}</span><span class="se">\n\n</span><span class="s2">data:</span><span class="se">\n</span><span class="si">{}</span><span class="s2">&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">_metadata</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">_t</span><span class="p">)</span>

    <span class="k">def</span> <span class="nf">__torch_function__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">func</span><span class="p">,</span> <span class="n">types</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(),</span> <span class="n">kwargs</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
        <span class="k">if</span> <span class="n">kwargs</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
            <span class="n">kwargs</span> <span class="o">=</span> <span class="p">{}</span>
        <span class="n">args</span> <span class="o">=</span> <span class="p">[</span><span class="n">a</span><span class="o">.</span><span class="n">_t</span> <span class="k">if</span> <span class="nb">hasattr</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="s1">&#39;_t&#39;</span><span class="p">)</span> <span class="k">else</span> <span class="n">a</span> <span class="k">for</span> <span class="n">a</span> <span class="ow">in</span> <span class="n">args</span><span class="p">]</span>
        <span class="n">ret</span> <span class="o">=</span> <span class="n">func</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
        <span class="k">return</span> <span class="n">MetadataTensor</span><span class="p">(</span><span class="n">ret</span><span class="p">,</span> <span class="n">metadata</span><span class="o">=</span><span class="bp">self</span><span class="o">.</span><span class="n">_metadata</span><span class="p">)</span>
</pre></div>
</div>
<p>This simple implementation won’t necessarily work with every function in the
<a class="reference internal" href="../torch.html#module-torch" title="torch"><code class="xref py py-mod docutils literal notranslate"><span class="pre">torch</span></code></a> API but it is good enough to capture most common operations:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">metadata</span> <span class="o">=</span> <span class="p">{</span><span class="s1">&#39;owner&#39;</span><span class="p">:</span> <span class="s1">&#39;Ministry of Silly Walks&#39;</span><span class="p">}</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">m</span> <span class="o">=</span> <span class="n">MetadataTensor</span><span class="p">([[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">],</span> <span class="p">[</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">]],</span> <span class="n">metadata</span><span class="o">=</span><span class="n">metadata</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">t</span> <span class="o">=</span> <span class="n">torch</span><span class="o">.</span><span class="n">tensor</span><span class="p">([[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">],</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">]]])</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">torch</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">t</span><span class="p">,</span> <span class="n">m</span><span class="p">)</span>
<span class="go">Metadata:</span>
<span class="go">{&#39;owner&#39;: &#39;Ministry of Silly Walks&#39;}</span>

<span class="go">data:</span>
<span class="go">tensor([[2, 4],</span>
<span class="go">        [4, 6]])</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">torch</span><span class="o">.</span><span class="n">mul</span><span class="p">(</span><span class="n">t</span><span class="p">,</span> <span class="n">m</span><span class="p">)</span>
<span class="go">Metadata:</span>
<span class="go">{&#39;owner&#39;: &#39;Ministry of Silly Walks&#39;}</span>

<span class="go">data:</span>
<span class="go">tensor([[1, 4],</span>
<span class="go">        [3, 8]])</span>
</pre></div>
</div>
</div>
<div class="section" id="operations-on-multiple-types-that-define-torch-function">
<h3>Operations on multiple types that define <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code><a class="headerlink" href="#operations-on-multiple-types-that-define-torch-function" title="Permalink to this headline">¶</a></h3>
<p>It is possible to use the torch API with multiple distinct types that each have
a <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> implementation, but special care must be taken. In such
a case the rules are:</p>
<ul class="simple">
<li><p>The dispatch operation gathers all distinct implementations of
<code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> for each operand and calls them in order: subclasses
before superclasses, and otherwise left to right in the operator expression.</p></li>
<li><p>If any value other than <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code> is returned, that value is
returned as the result. Implementations can register that they do not
implement an operation by returning <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>.</p></li>
<li><p>If all of the <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> implementations return
<code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>, PyTorch raises a <code class="docutils literal notranslate"><span class="pre">TypeError</span></code>.</p></li>
</ul>
</div>
<div class="section" id="testing-coverage-of-overrides-for-the-pytorch-api">
<h3>Testing Coverage of Overrides for the PyTorch API<a class="headerlink" href="#testing-coverage-of-overrides-for-the-pytorch-api" title="Permalink to this headline">¶</a></h3>
<p>One troublesome aspect of implementing <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> is that if some
operations do and others do not have overrides, users will at best see an
inconsistent experience, or at worst will see errors raised at runtime when they
use a function that does not have an override. To ease this process, PyTorch
provides a developer-facing API for ensuring full support for
<code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code> overrides. This API is private and may be subject to
changes without warning in the future.</p>
<p>First, to get a listing of all overridable functions, use
<code class="docutils literal notranslate"><span class="pre">torch._overrides.get_overridable_functions</span></code>. This returns a dictionary whose
keys are namespaces in the <code class="docutils literal notranslate"><span class="pre">PyTorch</span></code> Python API and whose values are a list of
functions in that namespace that can be overriden. For example, let’s print the
names of the first 5 functions in <code class="docutils literal notranslate"><span class="pre">torch.nn.functional</span></code> that can be
overriden:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">torch._overrides</span> <span class="kn">import</span> <span class="n">get_overridable_functions</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">func_dict</span> <span class="o">=</span> <span class="n">get_overridable_functions</span><span class="p">()</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">nn_funcs</span> <span class="o">=</span> <span class="n">func_dict</span><span class="p">[</span><span class="n">torch</span><span class="o">.</span><span class="n">nn</span><span class="o">.</span><span class="n">functional</span><span class="p">]</span>
<span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">([</span><span class="n">f</span><span class="o">.</span><span class="vm">__name__</span> <span class="k">for</span> <span class="n">f</span> <span class="ow">in</span> <span class="n">nn_funcs</span><span class="p">[:</span><span class="mi">5</span><span class="p">])</span>
<span class="go">[&#39;adaptive_avg_pool1d&#39;, &#39;adaptive_avg_pool2d&#39;, &#39;adaptive_avg_pool3d&#39;,</span>
<span class="go"> &#39;adaptive_max_pool1d&#39;, &#39;adaptive_max_pool1d_with_indices&#39;]</span>
</pre></div>
</div>
<p>This listing of functions makes it possible to iterate over all overridable
functions, however in practice this is not enough to write tests for all of
these functions without laboriously and manually copying the signature of each
function for each test. To ease this process, the
<code class="docutils literal notranslate"><span class="pre">torch._overrides.get_testing_overrides</span></code> function returns a dictionary mapping
overridable functions in the <code class="docutils literal notranslate"><span class="pre">PyTorch</span></code> API to dummy lambda functions that have
the same signature as the original function but unconditionally return -1. These
functions are most useful to use with <code class="docutils literal notranslate"><span class="pre">inspect</span></code> to analyze the function
signature of the original <code class="docutils literal notranslate"><span class="pre">PyTorch</span></code> function:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">import</span> <span class="nn">inspect</span>
<span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">torch._overrides</span> <span class="kn">import</span> <span class="n">get_testing_overrides</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">override_dict</span> <span class="o">=</span> <span class="n">get_testing_overrides</span><span class="p">()</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">dummy_add</span> <span class="o">=</span> <span class="n">override_dict</span><span class="p">[</span><span class="n">torch</span><span class="o">.</span><span class="n">add</span><span class="p">]</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">inspect</span><span class="o">.</span><span class="n">signature</span><span class="p">(</span><span class="n">dummy_add</span><span class="p">)</span>
<span class="go">&lt;Signature (input, other, out=None)&gt;</span>
</pre></div>
</div>
<p>Finally, <code class="docutils literal notranslate"><span class="pre">torch._overrides.get_ignored_functions</span></code> returns a tuple of functions
that explicitly cannot be overrided by <code class="docutils literal notranslate"><span class="pre">__torch_function__</span></code>. This list can be
useful to confirm that a function that isn’t present in the dictionary returned
by <code class="docutils literal notranslate"><span class="pre">get_overridable_functions</span></code> cannot be overriden.</p>
</div>
</div>
<div class="section" id="writing-custom-c-extensions">
<h2>Writing custom C++ extensions<a class="headerlink" href="#writing-custom-c-extensions" title="Permalink to this headline">¶</a></h2>
<p>See this
<a class="reference external" href="https://pytorch.org/tutorials/advanced/cpp_extension.html">PyTorch tutorial</a>
for a detailed explanation and examples.</p>
<p>Documentations are available at <a class="reference internal" href="../cpp_extension.html"><span class="doc">torch.utils.cpp_extension</span></a>.</p>
</div>
<div class="section" id="id1">
<h2>Writing custom C extensions<a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h2>
<p>Example available at
<a class="reference external" href="https://github.com/pytorch/extension-ffi">this GitHub repository</a>.</p>
</div>
</div>


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